In this article

General Information

Executive Summary

Microsoft is announcing the availability of an update for all supported editions of Windows 7 and Windows Server 2008 R2 to add support for SHA-2 signing and verification functionality. Windows 8, Windows 8.1, Windows Server 2012, Windows Server 2012 R2, Windows RT, and Windows RT 8.1 do not require this update as SHA-2 signing and verification functionality is already included in these operating systems. This update is not available for Windows Server 2003, Windows Vista, or Windows Server 2008.

Recommendation. Customers who have automatic updating enabled and configured to check online for updates from Microsoft Update typically will not need to take any action because this security update will be downloaded and installed automatically. Customers who have not enabled automatic updating need to check for updates and install this update manually. For information about specific configuration options in automatic updating, see Microsoft Knowledge Base Article 294871.

For administrators and enterprise installations, or end users who want to install this security update manually (including customers who have not enabled automatic updating), Microsoft recommends that customers apply the update at the earliest opportunity using update management software, or by checking for updates using the Microsoft Update service. The updates are also available via the download links in the Affected Software table in this advisory.

Advisory FAQ

What is the scope of the advisory?The purpose of this advisory is to inform customers of an update that adds functionality for the SHA-2 hashing algorithm to all supported editions of Windows 7 and Windows Server 2008 R2.

Is this a security vulnerability that requires Microsoft to issue a security update?No. A signing mechanism alternative to SHA-1 has been available for some time, and the use of SHA-1 as a hashing algorithm for signing purposes has been discouraged and is no longer a best practice. Microsoft recommends using the SHA-2 hashing algorithm instead and is releasing this update to enable customers to migrate digital certificate keys to the more secure SHA-2 hashing algorithm.

What is the cause of the problem with the SHA-1 hashing algorithm?The root cause of the problem is a known weakness of the SHA-1 hashing algorithm that exposes it to collision attacks. Such attacks could allow an attacker to generate additional certificates that have the same digital signature as an original. These issues are well understood and the use of SHA-1 certificates for specific purposes that require resistance against these attacks has been discouraged. At Microsoft, the Security Development Lifecycle has required Microsoft to no longer use the SHA-1 hashing algorithm as a default functionality in Microsoft software. For more information, see Microsoft Security Advisory 2880823 and the Windows PKI blog entry, SHA1 Deprecation Policy.

What does the update do?The update adds SHA-2 hashing algorithm signing and verification support to affected operating systems, which includes the following:

What is Secure Hash Algorithm (SHA-1)?The Secure Hash Algorithm (SHA) was developed for use with the Digital Signature Algorithm (DSA) or the Digital Signature Standard (DSS) and generates a 160-bit hash value. SHA-1 has known weaknesses that exposes it to collision attacks. Such attacks could allow an attacker to generate additional certificates that have the same digital signature as an original. For more information about SHA-1, see Hash and Signature Algorithms.

What is RFC3161?RFC3161 defines the Internet X.509 Public Key Infrastructure Time-Stamp Protocol (TSP) describing the format of requests and responses to a Time Stamping Authority (TSA). The TSA is can be used to prove that a digital signature was generated during the validity period of a public key certificate, see X.509 Public Key Infrastructure.

What is a digital certificate?In public key cryptography, one of the keys, known as the private key, must be kept secret. The other key, known as the public key, is intended to be shared with the world. However, there must be a way for the owner of the key to tell the world to whom the key belongs. Digital certificates provide a way to do this. A digital certificate is an electronic credential used to certify the online identities of individuals, organizations, and computers. Digital certificates contain a public key packaged together with information about it (who owns it, what it can be used for, when it expires, and so forth). For more information, see Understanding Public Key Cryptography and Digital Certificates.

What is the purpose of a digital certificate?Digital certificates are used primarily to verify the identity of a person or device, authenticate a service, or encrypt files. Normally, there is no need to think about certificates at all, aside from the occasional message stating that a certificate is expired or invalid. In such cases, one should follow the instructions provided in the message.

Suggested Actions

Apply the update for supported releases of Microsoft Windows

The majority of customers have automatic updating enabled and will not need to take any action because the update will be downloaded and installed automatically. Customers who have not enabled automatic updating need to check for updates and install this update manually. For information about specific configuration options in automatic updating, see Microsoft Knowledge Base Article 294871.

For administrators and enterprise installations, or end users who want to install this security update manually (including customers who have not enabled automatic updating), Microsoft recommends that customers apply the update at the earliest opportunity using update management software, or by checking for updates using the Microsoft Update service. The updates are also available via the download links in the Affected Software table in this advisory.

Users running Microsoft software should apply the latest Microsoft security updates to help make sure that their computers are as protected as possible. If you are not sure whether your software is up to date, visit Microsoft Update, scan your computer for available updates, and install any high-priority updates that are offered to you. If you have automatic updating enabled and configured to provide updates for Microsoft products, the updates are delivered to you when they are released, but you should verify that they are installed.

Other Information

Microsoft Active Protections Program (MAPP)

To improve security protections for customers, Microsoft provides vulnerability information to major security software providers in advance of each monthly security update release. Security software providers can then use this vulnerability information to provide updated protections to customers via their security software or devices, such as antivirus, network-based intrusion detection systems, or host-based intrusion prevention systems. To determine whether active protections are available from security software providers, please visit the active protections websites provided by program partners, listed in Microsoft Active Protections Program (MAPP) Partners.

Disclaimer

The information provided in this advisory is provided "as is" without warranty of any kind. Microsoft disclaims all warranties, either express or implied, including the warranties of merchantability and fitness for a particular purpose. In no event shall Microsoft Corporation or its suppliers be liable for any damages whatsoever including direct, indirect, incidental, consequential, loss of business profits or special damages, even if Microsoft Corporation or its suppliers have been advised of the possibility of such damages. Some states do not allow the exclusion or limitation of liability for consequential or incidental damages so the foregoing limitation may not apply.